The present invention relates to a new process for the preparation of urethane group-containing polyurea elastomers based on polyisocyanates or polyisocyanate mixtures of the diphenylmethane series, relatively high molecular weight polyhydroxyl compounds and alkyl substituted aromatic diamines by the reaction injection moulding technique.
The preparation of urethane group-containing polyurea elastomers based on polyisocyanates or polyisocyanate mixtures of the diphenylmethane series, relatively high molecular weight polyhydroxyl compounds and alkyl substituted aromatic diamines by the reaction injection moulding technique has already been disclosed in U.S. Pat. No. 4,218,543, which describes a one-shot process. In this process, the polyisocyanate components are mixed with compounds containing isocyanate reactive groups as well as additives. The process requires the use of catalysts for the reaction between isocyanate groups and hydroxyl groups.
It is known from German Offenlegungsschrift 3,827,595 (which corresponds to U.S. application Ser. No. 07/386,084, filed Jul. 28, 1989) that a virtually complete preliminary reaction of the relatively high molecular weight polyol component b) with the polyisocyanate component a) followed by reaction of the resulting isocyanate semiprepolymers with the diamine component leads to numerous remarkable advantages compared with the one-shot method, in particular as regards the mechanical and thermomechanical properties such as dimensional stability under heat in spite of large and differing volume streams (large volume of the isocyanate semiprepolymers, small volume of the diamine component).
The authors of the U.S. Pat. No. '595 reference, however, were obviously of the view that the preparation of high quality elastomers would only succeed if the relatively high molecular weight polyhydroxyl compounds b) had an average hydroxyl functionality of at least 2.5.
Although the process according to the U.S. Pat. No. '595 reference permits the production of elastomeric mouldings having a high level of mechanical properties, it has the disadvantage that the necessity to use branched polyhydroxyl compounds b) results in relatively high viscosities of the isocyanate semiprepolymers, so that the filling of large or complicated molds is in many cases difficult or impossible, especially when mineral reinforcing materials are used.
U.S. Pat. No. 4,297,444 describes the production of polyurethane moldings by the reaction injection molding technique using organic polyisocyanates, relatively high molecular weight polyhydroxyl compounds and low molecular weight chain lengthening agents. In the preferred embodiment, the total quantity of relatively high molecular weight polyhydroxyl component is first reacted with the polyisocyanate component to produce an isocyanate semiprepolymer, but this prior publication gives no indication that systems utilizing a diamine component which consists of alkyl substituted aromatic diamines, could also be processed on this principle or that any advantages would result from such a process. Although aromatic diamines are mentioned by the way in the list of suitable chain lengthening agents, the use thereof is not further discussed. In fact, the chain lengthening agents mentioned as preferred are simple diols such as ethylene glycol or 1,4-butanediol. Only ethylene glycol is used in the examples. The possibility that reactive systems consisting of a large volume stream of isocyanate semiprepolymer and a small volume stream of highly reactive diamines could be processed by the reaction injection molding technique thus could not be deduced from the teaching of this reference.
German Auslegeschrift 1,240,654 describes the use of monomer-free, i.e. true isocyanate prepolymers, based on polyisocyanates other than those of the type to be used according to the invention and difunctional polyhydroxyl compounds, which are reacted with alkyl substituted diamines, preferably in the presence of solvents, by the classical prepolymer process and by no means on the principle of the reaction injection moulding technique.
Published European patent application 0,075,130 describes a process similar to that described in U.S. Pat. No. 4,218,543, where certain prepolymers are used. However, the reference generally describes using a significant amount of high molecular weight polyol in the B-side.